Philcoxia couldn’t look more unassuming. It’s a small herb that lives in Brazil’s Campos Rupestres region, a sparse plateau of rocky outcrops and white sands. All you’d see of it are a handful of twigs sticking out from the grains, topped with small purple flowers and even smaller leaves. You wouldn’t think that it’s the type of plant that can kill animals.

To find Philcoxia’s grisly secret, Caio Pereira had to look underground. The plant biologist from Unicamp, Brazil, found that the planttraps and digests tiny worms with sticky underground leaves.

Plants have evolved to eat animals at least six times, and over 600 species of them now do so. They catch their prey with slipperywater-filled pitchers, fast-snapping traps, sticky leaves and sucking bladders. Their strategies are diverse, but they all tend to grow in areas that are poor in nutrients. For example, familiar species like the Venus’ fly trap and the sundew live in bogs and swamps. In such inhospitable environments, these plants supplement their supplies of nitrogen, phosphorus, and other nutrients by feasting on the flesh of animals.

Philcoxia is no different. When Peter Taylor described the plants in 2000, he noted that they have several features that resemble those of other meat-eating species. Its mountain home is poor in nutrients and frequently starved of rain. Its roots are unusually sparse and lack the fungal partners that help other plants to survive. And it has tiny underground leaves – just a millimetre wide and coated with sticky glands, of the type found in other carnivorous plants.

There was just one problem. There were no signs of captured animals, or bodies nearby. If Philcoxia ate animals, it wasn’t doing so obviously. In 2007, Peter Fritsch found a possible answer. He noticed nematode worms stuck to the underground leaves, and reasoned that the plant was trapping and digesting them. Pereira, working with Fritsch, has now confirmed this hypothesis.

He found that Philcoxia’s underground leaves are littered with the bodies of dead nematodes. To check that the deaths aren’t coincidental, Pereira bred nematodes so that their bodies were full of nitrogen-15 – a rare and heavier-than-usual version of the element. He then “fed” the nematodes to Philcoxia. Two days later, Pereira found that 15 percent of the nitrogen-15 in the worms has been incorporated into the plant’s leaves. It was clear proof that Philcoxia was digesting the nematodes and absorbing the remains into their bodies.

Many meat-eating plants digest their prey with high concentrations of enzymes called phosphatases. Philcoxia does so too. Pereira found loads of the enzymes on Philcoxia’s leaves, which means that the plants are probably digesting the nematodes directly.

Pereira still wants to find out how Philcoxia lures its prey towards its traps. But for the moment, he has shown that its diet of worms greatly raises the level of nitrogen in itsleaves, boosting it even above the levels of neighbouring plants that aren’t carnivorous.

That’s interesting, because some scientists have suggested that eating animals is a relatively inefficient way of coping with nutrient-poor environments. This would explain why carnivorous plants are relatively rare. But Philcoxia clearly shows that a fleshy menu can provide more nutrients the strategies of other plants in the same conditions.

Indeed, Philcoxia’s murderous habits suggest that we may have underestimated the true number of meat-eating plants in the world. After all, if this rare species feeds on microscopic prey using hidden traps, perhaps other plants do so too. As Mark Chase wrote a few years back, “we may be surrounded by many more murderous plants than we think”.

Related

I’m having trouble following the premise to the conclusion. If I fed you an abundance of nothing but Cadbury Chocolates, would you not have a stomach full of chocolate when I peeked inside you? This does not prove that you live on chocolate even if I have seen you eat one or two in the past. How was the experiment set up to show that the mechanism he used would be what the plant would experience in its natural setting?

The whole point of the article is that it wasn’t yet clear that this plant can actually trap and eat prey. You can’t just “feed” nematodes to a tomato plant because tomato plants don’t have a mechanism for trapping and digesting them. Therefore the isotope nitrogen-15 wouldn’t show up in the tomato plant in any significant amount.

There was already an abundance of evidence that the plants were probably eating nematodes, including repeated field observations of nematodes stuck to its leaves. The nitrogen experiment confirms that they have means of breaking down the nematodes stuck to their leaves and absorbing the resulting nutrients.

I think the research gets us a little closer to calling these carnivorous plants, especially the evidence that they do produce phosphatases. But the logical leap I can’t make with the authors of the study is that the plant alone was responsible for digestion. I’m not all that impressed by foliar absorption of the nitrogen-15 since foliar absorption of mineralized nutrients is common. What strikes me is that they left the nematodes on the leaf for 48 hr where surface bacteria would have more than enough time to mineralize nutrients. Their data only suggests that the plant’s enzymes were the ones largely responsible, and it’s an easy jump to make. The criticism that was laid at the feet of other studies suggesting plants such as Stylidium were carnivorous is the same here: it wasn’t a sterile environment (e.g. tissue culture), so how can we know digestion was primarily the plant’s doing? I’m also looking forward to the author’s promised data on photosynthetic rates to explicitly show whether the plants are deriving a benefit from the nematodes.

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Phenomena is a gathering of spirited science writers who take delight in the new, the strange, the beautiful and awe-inspiring details of our world. Phenomena is hosted by National Geographic magazine, which invites you to join the conversation. Follow on Twitter at @natgeoscience.

Ed Yong is an award-winning British science writer. Not Exactly Rocket Science is his hub for talking about the awe-inspiring, beautiful and quirky world of science to as many people as possible.
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